Air exchange rates and advection–diffusion of CO2 and aerosols in a route bus for evaluation of infection risk

Abstract As COVID‐19 continues to spread, infection risk on public transport is concerning. Air exchange rates (ACH) and advection–diffusion of CO2 and particles were determined in a route bus to evaluate the infection risk. ACH increased with bus speed whether windows were open or closed, and ACH were greater when more windows were open. With two open windows, ACH was greater when a front and rear window were open than when two rear windows were open. With both front and rear ventilation fans set to exhaust, ACH was more than double that when both were set to supply. With air conditioning (AC) off, CO2 and particles spread proportionally at the same rate from a source, whereas with the AC on, the spread rate of particles was about half that of CO2, because particles might be trapped by a prefilter on the AC unit. Infection risk can be reduced by equipping AC unit with an appropriate filter. Calculations with a modified Wells–Riley equation showed that average infection risk was reduced by 92% in the moving bus with windows open comparing to with windows closed. When the bus was moving with windows closed, exhaust fan operation reduced the average risk by 35%.

is necessary to understand ventilation conditions and airflow and particle behaviors in the space of interest.
Many studies have investigated airflow and particle behaviors in mobile vehicles transporting passengers, but the majority of these have been numerical simulations. [11][12][13] For example, although an aircraft cabin is an enclosed space, approximately 50% of the cabin air is derived from outdoor air; further, all the cabin air is exchanged with outdoor air every 3-4 min, and the recirculated air is cleaned by passing through a HEPA filter, which can remove 99.97% of 0.1-0.3 μm particles. 14,15 Compared to an aircraft cabin, in buses the performance of ventilation fans and air conditioning filters has not been well investigated. Although computational fluid dynamics (CFD) analyses 16,17 and particle measurements 17 have been conducted to investigate the diffusion of droplets in a stationary bus, no studies have obtained empirical evidence to evaluate airflow in actual moving buses.
The SARS-CoV-2 outbreaks in tourist buses have been reported, 18 but no such outbreaks have been reported for route buses.
The arrangement of the doors and windows and vehicle operation of tourist buses are quite different from those of route buses.
In a case-control study conducted during influenza season, Troko et al 19  transmission based on measured ACH in subway trains, but they did not measure the spread of aerosol particles such as smaller droplets and droplet nuclei. Moreover, unlike subway trains, buses are equipped with ventilation fans, which may lead to different ventilation behavior.
In this study, to obtain empirical data on the effectiveness of countermeasures that would contribute to infection control in route buses, ACH in a bus were determined under various conditions by using the CO 2 decay method. In addition, to simulate the advection-diffusion of droplet nuclei from an infected individual, CO 2 and particles were emitted at one point in the bus, and the spread of gaseous CO 2 and particles in the bus from that point was measured and evaluated. Then, the infection risk in the bus was evaluated and discussed.

| Survey period and bus characteristics
During 1-5 and 28-30 August 2020, ACH were determined in a large route bus (Isuzu LV290Q1; Figure S1). Stationary tests were con-   Figure S2). Two doors were located on the left side of the bus. Two ventilation fans, both of which could be set to either supply or exhaust air, were located in the center of the ceiling near the front and toward the back of the bus ( Figure S2). Two air conditioner (AC) suction inlets (return vents) were located in the ceiling at the center front of the bus, and 38 AC outlets (supply vents) were located in the ceiling above passenger seats ( Figure S2). The total flow rate of the AC in the bus was 3850 m 3 /h, which means that air equivalent to the room volume can be circulated 77 times in an hour. A coarse dust filter was set in the AC unit.

| Determination of air exchange rates
ACH were determined by the CO 2 decay method, 27,28 which is appropriate for obtaining short-term ACH in an unoccupied space.
After CO 2 emission from gas cylinders in the bus, in-bus air was agitated by four circulators (EAC-23-W, Iris Ohyama Inc., Sendai, Japan) and two blowers (Earthman BW-144LiAX, Takagi Co., Ltd., Kitakyushu, Japan) until the CO 2 concentration reached equilibrium across all sampling points. The in-bus CO 2 concentration was monitored with CO 2 /humidity/temperature data recorders (MCH-383SD, Lutron Electronic Enterprise Co., Taipei, Taiwan) every 10 s for 15 min to 1 h at 24 points in the bus ( Figure 1B); 10 monitoring points were at a height of 1.5 m (to estimate the exposure of standing passengers) and 14 were at a height of 0.7 m above the seat level (sitting passengers) to match the height of the passenger's mouth. The outdoor CO 2 concentration was measured with the same CO 2 instruments at the same time at a height of 1.5 m above the ground within 100 m of the bus and out of direct sunlight. In-bus and outdoor temperatures were also measured with the same instruments. ACH were calculated by fitting an exponential function to differences in the monitored CO 2 concentrations between indoor and outdoor environments. Before the survey, all of the CO 2 recorders were calibrated to several different concentrations of CO 2 gas, including atmospheric level, measured by a previously calibrated CO 2 monitor (Model 2211, Kanomax Japan, Inc., Japan).
To evaluate differences in the ACH due to different vehicle speeds, AC operation, the presence or absence of passengers, operation of ventilation fans, and open or closed windows, the CO 2 concentration decay was determined both while the bus was stationary (0 km/h) and while it was moving at 10, 20, or 30 km/h, with the AC off or on, with or without passengers (mannequins), with the ventilation fans off or on, and with the windows closed or open (Table S1). All test conditions were tested once. The upper limit of vehicle speed was set at 30 km/h to simulate rush hour traffic, and speeds of 10 and 20 km/h were considered to simulate the slower vehicle speeds seen in large cities since the average moving speed of vehicles during rush hour was 14.3-15.4 km/h in Tokyo and 16.9-35.6 km/h in whole Japan 2015. 29 To represent passengers, 75 mannequins (25 sitting mannequins and 50 standing mannequins) made of resin without heat were placed in the bus to simulate a full cabin.
While the AC was on, the temperature was set to 26°C. In the open windows test, both sides of each of the five windows were opened to the maximum amount ( Figure S3). In addition, we conducted a test without mannequins to simulate typical operation of a route bus; in this test, the bus repeatedly moved 500 m at 30 km/h and then To evaluate the effect of ventilation fan operation, the CO 2 concentration decay was measured under four different conditions by separately setting the front and rear ventilation fans to either exhaust or supply mode.

| Advection-diffusion of artificial droplet nuclei
To evaluate the advection-diffusion of droplet nuclei from an infected individual in the bus, CO 2 or particles were emitted at a constant volume flow rate from a single location and measured at 24 (CO 2 ) and 5 points (particles). For CO 2 ( Figure 1B), the emission point was either at location A at the rearmost right-side seat, simulating a sitting passenger facing the front, or at location B in the middle near the second frontmost right-side seat, simulating a standing passenger facing the window. For particles, the emission point was only at location A in Figure 1C. Every test condition is listed in (Table S2).  Figure 1C).
In the laboratory before the survey, the line from an atomizer was divided six ways and simultaneously connected to the six OPSs, and the emitted particles were measured. The relative values were used to correct for differences among the OPSs.
In the first test, CO 2 and particles were emitted at location A ( Figure 1B and C) for 15 min, and in the second test, CO 2 was emitted at location B ( Figure 1B) Figure 3D).
In Figure 3B ( and with the AC off, compared to 52/h with mannequins and the AC off ( Figure 3C and D).

| Advection-diffusion of CO 2 and particles
The CO 2 concentration changes over 15 min in each seat are visualized in movies (Movies S1-S3). When the AC was off, CO 2 emitted from the right rear seat spread forward via the center aisle, except when the rear ventilation fan was set to exhaust mode. When the rear ventilation fan was set to exhaust mode, it spread across to the opposite side of the bus and then forward. CO 2 emitted from the front standing position spread toward the windows and then was reflected back and spread in the direction opposite to the emission direction. There was a noticeable tendency for the air to flow toward the front of the bus when the front fan was set to exhaust and the rear fan was set to supply, and for the air to flow toward the back of the bus when the front fan was set to supply and the rear fan was set to exhaust.
In stationary tests with mannequins, CO 2 concentration was higher near the source and lower away from the source under all conditions ( Figure 6), but the CO 2 concentration tended to be reduced overall when windows were open (rows i → iii, ii → iv) or the ventilation fans were on (columns I → II-IV). When CO 2 was emitted at the rearmost seat with the windows closed (the upper two rows (i and ii) in Figure 6A), the CO 2 concentration varied more by location with the AC off (coefficient of variation [CV] 43-140% (row ii)) than when the AC was on (CV 30-90% (row i)). When CO 2 was emitted at the front standing position with the windows closed ( Figure 6B In addition, the CO 2 concentration in the bus in the stop & go tests with the windows closed (0.093 and 0.10% of P CO2 , Figure 7B rows i, ii and column I) was decreased by 26% and 27% compared to that in the moving bus at 30 km/h with the windows closed (0.13 and 0.13% of P CO2 , Figure 7A rows i, ii and column I).

Concentration profiles for particles on positions and window
open or closed were similar to those for CO 2 , that is, higher at points (1)-(3) than (4) and (5) and higher with the windows open than with the windows closed ( Figure S5). However, the concentration trends for particles on AC operation was different from CO 2 , that is, lower with the AC on than with the AC off ( Figure S5). With the AC off, P CO2 showed an approximately 1:1 correlation with P particles ( Figure 8B), whereas with the AC on ( Figure 8A), the slope of the P particle -P CO2 relationship was approximately 0.5. With the AC on, the slope of P particle -P CO2 relationship was smallest at point (5) which is the only point farther from the source than the AC suction inlets ( Figure 8A).

| DISCUSS ION
When the bus was moving continuously without stopping and it can be assumed that the ACH would increase further as the bus speed increased above 30 km/h. However, further research is required on the relationships between C-value and ACH and between ACH and speed.
The average vehicle speed of route buses varies greatly among urban, suburban, and rural areas, and between commuting hours and other times of the day. Therefore, not only the number of passengers and the distance between bus stops but also the average speed of the vehicle, which varies depending on the location and time of day, must be taken into consideration when implementing infection control countermeasures. In addition, because the ACH varies with vehicle speed, it is desirable to ensure a certain level of ACH regardless of speed. Therefore, buses should be equipped with a ventilation system with a capacity sufficient to achieve the desire ACH, and/or the AC system should be equipped with filters that can reduce the concentration of airborne droplet nuclei. Further research is needed to assess risk reduction by filters.
Ventilation fan operation (front, supply; rear, exhaust) had no effect when the windows were open and the bus was moving (compare Figure 3 (C) and Figure S4). This suggested that when the bus   Figure 5). The decrease in the airflow volume during actual operation compared to the ventilation fan capacity might be due to a decrease in the pressure difference between the inside and outside of the bus (i.e., resistance to air being pressed/sucked in via gaps in the exterior surface of the bus). In general, pressure on the roofs of high buildings is negative relative to the atmospheric pressure, 33 and a simulation result has suggested that when a bus is moving, pressure on the roof of the bus will also be negative relative to the surrounding air pressure. 34 Unless there is a strong wind, the wind speed above the roof of a bus should not be high unlike a high building, so pressure on the bus roof is not negative when the bus is stationary. However, in this study, the ACH did not differ between the stationary and moving tests with the ventilation fans on; this result suggests that the contribution to ventilation of negative pressure on the roof is small, even when the bus is moving, and the contribution of static pressure inside the bus is large. The ACH was greater when both front and rear fans were set on exhaust than when both fans were set on supply because of characteristic P-Q curves (where P is static pressure and Q is the airflow rate) of the fans, which we obtained from the manufacturer (data not shown); namely, the airflow rate in the exhaust direction becomes greater than that in the supply direction for any static pressure value above zero. As a result, when both the front and rear fans were set on exhaust, the static pressure in the bus (calculated to be 28 Pa) was higher than that when both front and rear fans were set on supply (calculated to be 14-15 Pa). This suggests that the difference in the ACH between air supply and exhaust was due to the difference in the static pressure-air volume characteristics of the fans installed in this bus between exhaust and supply due to the fan's design.
The more windows were open in the bus, the greater ACH was observed. The same trend was observed on subway trains in the present study. 26 While the bus was moving with two windows F I G U R E 6 (A) Inhalation volume of CO 2 relative to the volume emitted in the right rearmost part of the bus (P CO2 ), in stationary tests with mannequins. Average of surrounded data was used for two points whose data were lost. (B) Inhalation volume of CO 2 relative to the volume emitted in the front center part (standing) of the bus (P CO2 ), in stationary tests with mannequins. Average of surrounded data was used for a point whose data were lost When the windows were closed and the ventilation fans were off and CO 2 was emitted from a single location and spread in the bus by advection and diffusion, the maximum P CO2 near the source was 0.29-0.40% if the bus was stationary and 0. 16 ized exposure for each seat at 3-min intervals over 15-min period (0-3, 3-6, 6-9, 9-12, and 12-15 min) and for the total 15-min period ( Figure 9). The CO 2 exposure at seat where CO 2 exposure was maximum in the bus during the 15 min was fixed to 100%, and then, the CO 2 exposure at other seats and for other time durations was calculated relative to that maximum. The results showed that even when the AC was turned off, a plume could be detected only within about three seats from the source ( Figure S6).
The correlation between P particles and P CO2 when CO 2 and particles were emitted at a single location was about 1:1 when the AC was off ( Figure 8B), but when the AC was on, P particles was lower than P CO2 although they were correlated ( Figure 8A). In particular, when the AC was on, the ratio of P particles to P CO2 at point (5) (see Figure 1C), the farthest point from the particle source, was lower than the ratio at points (1)-(4). This result implies that when the AC is on, some particles are removed from the air by being deposited on the wall because of the wind from AC or trapped in the AC dust filter.

Further investigation of particle deposition on walls and collection
on filters is required.
We used the following modified version of the and ~19% for ~570 /h and between ~53% and ~88% for ~5600/h).
Although the reduction in risk of infection achieved by opening the windows is not much different from that for COVID-19, additional countermeasures are required for vehicles where measles infection is a concern because of its high infection risk.
It has been suggested that a SARS-CoV-2 transmission cluster in a tourist bus in China may have been caused by airborne transmission to seats distant from the infected person via AC air circulation. 18 Because the bus used in the present study had many AC outlets (air supply vents) above the passengers, it would be possible for the virus to be spread from the infected person to other areas of the bus by the AC air circulation. However, we found that when the AC was on, the average CO 2 exposure concentration sometimes increased and sometimes decreased, depending on other conditions, but the average particle exposure concentration was decreased to approximately less than half; this result suggests that filtration through even a coarse dust filter could reduce the infection risk when the AC is on. By improving the performance of the AC filters, it might be possible to further reduce the risk. Furthermore, the present study results suggest that the risk was higher at locations between the AC inlet and the infected person (points (1)-(4) in Figure 1) than at locations beyond the AC inlet from the infected person (point (5)). This result indicates that the risk could be further reduced by having the AC inlets at multiple locations in the bus rather than at a single location.
However, when CO 2 and particles were emitted at the level of a standing passenger, the exposure concentration in the seat in front of the emission location tended to be higher when the AC was on, possibly owing to the effect of downward airflow from the AC. Thus, the degree of risk may depend on how droplet nuclei and microdroplet are spread in the bus by the AC airflow, regardless of which seat the infected person sits in. Therefore, further study of how AC circulation and particle removal by filters affect exposure is necessary.
The route bus used in this study was the standard type produced There is both temporal and spatial uncertainty in the results of the measurement of CO 2 /particle spread in the bus. They are caused by variance/error in CO 2 /particle emission/measurement and variance of airflow turbulence. Although it is possible to reduce the uncertainty by the repeated measurements, the test was conducted in a single session for each condition in the present research since it was hard to conduct multiple tests in this study according to the restrictions of bus usage and location. In addition, although the air exchange rates can vary depending on the outdoor air conditions (indoor-outdoor temperature difference, outdoor wind speed, etc.), these effects were also not evaluated in this study. The coefficient of variation (CV) of CO 2 and particle emission during the advectiondiffusion in test in this study was 0.85% ± 0.51% and 8.4% ± 3.7% (mean ± SD), respectively. The CV of the instrumental error was 0.37% (7000 ppm) to 6.8% (450 ppm) and 2.3% ± 2.2% for CO 2 and particle measurement, respectively. Although the eddy generated by the airflow from the emission source might be randomly generated, the uncertainty cannot be evaluated in the present measurement. It is necessary to evaluate the uncertainty of the results in the present research by simulating the behavior of CO 2 /particles with CFD simulations in the future.

| CON CLUS ION
The and that when the windows are closed, the risk of infection can be reduced by setting both front and rear ventilation fans to exhaust (35% in a moving bus). Although the risk is possible to be reduced by using even a coarse dust filter, a better-performing filter can be effective in reducing risk, especially when the AC is to be operated.